Contemporary railroad data processing systems require information relating to the number of railroad cars in a particular place or in a particular train. Presently, such information may be derived from trackside wheel detectors which count the number of wheels passing along the adjacent rail and divided by the proper constant to obtain the number of cars. There are currently a number of different types of wheel detectors available each of which has significant shortcomings. In one type a primary magnet provides magnetic flux through a switch and the neighboring rail. A secondary magnet provides through the switch an offsetting flux normally insufficient to overcome the primary flux. The switch normally resides in a first position. Upon arrival of a railroad car the magnetic mass of a wheel enters the primary magnetic field reducing the flux through the switch and enabling the secondary flux to overcome the primary flux in the switch and drive the switch to its second position. This type of wheel detector is expensive, complex and uses a great number of parts. The second type of wheel detector uses three a.c. energized coils. As the wheel passes over them the disturbance of the flux pattern causes a phase shift between the coils which can be detected as a wheel-passing. This device must be carefully monitored. Another type uses a magnetic reed switch held in one position by magnetic flux and allowed to switch to a second position when a wheel interrupts the flux pattern. This device is primarily mechanical in nature and suffers the same reliability shortcomings as other mechanical devices. Yet another device uses a transmitter on one side of a rail and a receiver on the other. The improved reception brought about by the presence of a wheel is detected. This type is quite complex and expensive. Still other detectors employ a latch that is physically tripped by the wheel.